The present invention relates to floating gate transistors and more particularly to floating gate metal-oxide-semiconductor transistors.
Floating gate metal-oxide-semiconductor (MOS) transistors are devices with a first gate situated on a layer of insulating oxide directly above the transistor channel, and a second control gate situated on a layer of insulating oxide above the first gate. The first gate is electrically isolated, hence the term ‘floating gate’, and any charge deposited on the floating gate will be held almost indefinitely. Floating gate devices are commonly used in digital integrated circuit (IC) design as the storage components of FLASH EPROM. Typically, floating gate MOS transistors are fabricated using thin film technology, with a high degree of integration between transistors and other components.
During the last decade, floating gate devices have also been exploited in analogue IC design to implement circuits and small systems. The availability of a floating gate device, where the charge (or voltage) on the floating gate can be controlled, leads to a number of useful circuit applications, including:                Analogue memory: Floating gate devices can be used as ‘analogue PROM’, particularly for applications such as neural networks which require non-volatile analogue storage.        Tuning: Any charge stored on the floating gate will affect the threshold voltage of the transistor. Thus transistors can effectively be tuned to ensure their threshold voltages are equal, by carefully controlling the charge stored on the floating gate. A further example is the auto-zero floating gate amplifier (AFGA), where the operating point at the input of the amplifier can be set by tuning a floating gate structure.        Level shifting: The floating gate is also a level shifter. Whatever charge is stored on the floating gate will add to the voltage applied on the control gate. This level shifting may be used (for example) for threshold shifting for tow-power low voltage circuits. With a preset voltage offset, the functional threshold of the transistor is changed accordingly.        Computation: The use of the floating gate transistor as a computational element is very attractive for low power analogue integrated circuit design. If the top (control) gate is divided into a number of smaller gates with scaled areas, the floating gate device effectively performs a weighted summation of the voltages applied on each of these top gates. By scaling the top gate areas (i.e. capacitor sizes) voltages can be weighted, relative to the capacitor sizes. Such a structure is indicated diagrammatically in FIG. 1. FIG. 1a represents a stacked floating gate device, where the top control gate (or gates) is (or are) situated directly above the channel region of the transistor. However such a stacked floating gate structure is not recommended, as the process steps required to fabricate the top gate can cause changes in the floating gate and substrate which affect the transistor's threshold voltage. The preferred architecture is shown in FIG. 1b, where the floating gate is extended laterally from the channel, and the top gate is deposited over this extended region.        
An informative review of both digital and analogue applications based on floating gates can be found in: IEEE Transaction on Circuits and Systems—Part II, Special Issue on Floating Gate Circuits and Systems, January 2001.
Despite the obvious attractions, the usage of floating gate structures in analogue circuits has not really taken off, due to a number of practical problems in the implementation of analogue floating gate circuits. In particular, programming and control of the charge on the floating gate is very difficult, and typically involves a combination of Fowler-Nordheim tunnelling and hot carrier injection. Both of these processes typically require the application of large voltages which allow electrons with sufficient energy to tunnel through the insulating silicon dioxide to and/or from the floating gate, thus altering the net charge on the gate. Although this process is similar to the methods used for programming and erasing digital ROMs, repeated programming causes degradation of the silicon oxide leading to transistor breakdown. This leads to long-term reliability problems for circuits which require constant tuning or alteration. In addition the application of high voltages to perform tuning is itself undesirable.
Another problem with floating gate devices is that the long-term charge storage capabilities are uncertain—over time the charge stored on the floating gate may slowly leak away. This problem will only get worse as process dimensions shrink and the oxide thickness reduces. Any slight charge leakage in a digital memory may not be a problem but is much more significant when an analogue value is being stored. This uncertainty in long-term charge storage has contributed to the reluctance to exploit floating gates in analogue ICs for commercial applications.
Recently, a mechanism for overcoming the above-mentioned problem in floating gate analogue circuit design—i.e. the difficulty in manipulating the charge stored on the floating gate—was proposed at IEEE Int. Symposium on Circuits and Syst. (ISCAS) 2001, Pre-Conference Workshops: ‘Multiple-Input Floating-Gate MOS Transistors as Functional Devices to Build Computing Circuits’ Tadashi Shibata, ‘Voltage-Mode Floating Gate Circuits’, Jaime Ramirez-Angulo. The mechanism involves the deliberate addition of a small leakage path to the floating gate (although in this architecture the gate is no longer truly “floating”, that terminology is still used for convenience). Such a leakage path could be provided using a high resistance value implemented using conventional CMOS resistors. However, this would require a prohibitively large silicon area, and thus is impractical. Shibata and Ramirez-Angulo therefore propose providing the leakage path via the addition of a single pull-up reverse biased diode clamping the floating gate potential to the positive supply rail, as shown in FIG. 2. The reverse-biased diode effectively acts as a very large resistor which pulls the voltage on the floating gate toward the voltage applied to the other end of the diode (in this case, the supply voltage). The voltage on the floating gate will respond to voltages applied to the top gate(s), deviating from the voltage of the positive supply rail for as long as the gate voltage(s) is(are) applied.
This simple mechanism, however, has severe limitations. Input signal perturbations of sufficient amplitude will forward bias the pull-up diode, introducing severe distortion into the device performance. Restricting the signal swing to well below one diode offset should reduce this effect, but the forward conduction problem can never be eliminated entirely.